The invention relates to an electromagnetic absorber material, to a method for producing it as well as to a method for producing shielding devices, such as absorber walls, absorber linings or absorber housings, including so-called anechoic measurement chambers, which are free of electromagnetic fields, using these absorber materials.
The invention is directed to the creation of conditions, as free as possible of fields, for carrying out especially accurate and/or particularly sensitive electrical measurements, as well as to protecting the population and, in particular, the employees during the commercial use of alternating electromagnetic fields against possible harmful effects.
It can be inferred from the discussion of the concept of xe2x80x9celectrosmogxe2x80x9d that the population has been sensitized to the industrially produced accumulation of electromagnetic radiation in the natural environment. Legislators as well as trade associations have reacted with enacting or tightening regulations concerning the maximum permissible power density of a source of radiation. The 26th BimSchV (Regulation Concerning Electromagnetic Fields) and the DIN VDE 0848 (Safety in Electromagnetic Fields) are mentioned as examples here. The limiting values, set down in the 26th BimSchV to protect the population, are based on international recommendations, such as the International Commission for Protection against Nonionizing Radiation (ICNIRP) or the World Health Organization (WHO). These recommendations are revised whenever new scientific results are obtained. The most recent publication of the ICNIRP of April 1998 confirms the values, which form the basis for the 26th BimSchV.
The limiting DIN VDE 0848 values for high frequency fields, permissible for the general population, are given in Table 1.
The division of the regulation into electrical and magnetic field components is due to the considerable expense of measuring the specific absorption rate (SAR(W/kg)). The SAR is the basic quantity for thermal effects, which is recognized worldwide, since the beam power, absorbed by the body, determines the biological effect of HF radiation.
According to current findings, SAR values of 1 to 4 W/kg (averaged over the whole body) lead to an increase in body temperature of 1xc2x0 C. in man within thirty minutes. A limiting, whole body, SAR value of 0.4 W/kg was fixed for protecting professionally exposed persons and a value of 0.08 W/kg was fixed for protecting the general population.
The problem of these limiting or precautionary values consists therein that, merely by the promulgation, a hazard is suggested to the population or the population is sensitized to a long-term effect, which is possibly not yet confirmed with certainty. This is reinforced further by the strictly technical discussion, that is, the exclusive reference to thermal effects. WHO, ICNIRP and IEGMP studies, currently taking place or already published, especially of the effects of using mobile phones on brain currents due to the frequency and modulation of the radiation of such phones, are pushing the so-called athermal effects increasingly into the foreground. For this reason and in expectation of positive results of studies taking place at the present time, in the sense of affecting the human body, the development of an absorber granulate for protecting the resident population has become an urgent necessity.
Predominantly ferrites and/or conductive substances in different mixtures and matrices are obtainable for absorbing electromagnetic waves. For example, Ferrite Domen Co. in Russia is offering ferrites in powder form as a microwave absorber; they have a useful frequency of 1 to a maximum of 40 GHz. Spectro Dynamic Systems in the USA is selling silver-coated Cenospheres for HF absorption. The latter are to be used as fillers in paint and resins for producing surface coatings. In an example, the damping performance is stated to be 60 dB from 100 MHz to 10 GHz for a film that is 5 mm thick. TDK is offering a range of radio-wave absorbers with a reflection damping in excess of 20 dB, a range from 0.03 to 40 GHz being covered. A further absorber is sold by Emerson and Cuming Microwave Products, Inc. under the name of ECCOSORB(copyright)MCS for frequencies ranging from 1 to 8 GHz with a damping of 6 to 63 dB/cm.
In the German Offenlegungsschrift 199 49 631 A1, a composite absorber for electromagnetic waves is described, for which a ferrite powder with a dielectric constant not greater than 4.9 is dispersed in a conventional resin, shaped into a pyramidal absorber and combined with a ferrite plate. The main components of the ferrite plate are stated to be Fe2O3, NiO, ZnO and CuO and the main components of the resin-bound pyramidal absorber are stated to be Fe2O3, NiO, and ZnO. At frequencies ranging from 100 MHz to 10 GHz, the achievable damping is stated to be at least 20 dB.
The German Offenlegungsschrift 195 25 636 A1 discloses a wall coating for absorbing electromagnetic waves. This wall coating makes possible a broadband reflection by the composite of a ferrite plate with a resistance material, facing the wall. Measured values for the reflection damping achieved are not given.
U.S. Pat. No. 5,323,160 discloses the production of an absorber by combining two soft ferrites (Mnxe2x80x94Zn, Nixe2x80x94Zn) to form layers of different thicknesses. In every case, these layers are applied on a metal as support. Measurements reveal damping of at least 20 dB at frequency ranging from 200 MHz to 1 GHz.
U.S. Pat. No. 5,446,459 discloses a broadband absorber consisting of a sintered ferrite and a CuOxe2x80x94Fe2O3 spinell ferrite. The absorption is measured with the help of an HP 8510A Network Analyzer using a coaxial measuring lead. Frequency ranges with damping in excess of 20 dB are claimed for different compositions. These frequency ranges are between a minimum of 98 MHz and a maximum of 950 MHz.
The publication EP 0 858 982 A1 discloses a composition for an absorber and the method for producing it. The absorber is composed of a mixture of Fe2O3, NiO, ZnO and CuO, which is ground, molded and sintered. The absorption rates are measured by means of the Holaday HI-400 RF Measuring System at different distances from a mobile phone. The absorption rates are given as percentages.
The German Offenlegungsschrift DE 199 11 304 A1 discloses a coating or film for shielding electromagnetically over a wide range of frequencies. For this purpose, a ferrite powder is mixed with a conductive powder and processed with the help of a paintable binder into films or coatings. The measured damping values are stated to be in excess of 30 dB/mm.
The state of the art, described above, suffers from the shortcoming that, using in some cases expensive methods and materials, only the aspect of an electromagnetic shielding is taken into consideration without regard to the applicability of these technologies in construction.
It is therefore an object of the invention to decrease the electromagnetic radiation effectively in a broad frequency band, preferably from 100 MHz to 10 GHz by means of an absorber material, this material combining the desired shielding properties with the thermal insulation properties of a porous material as well as with the advantageous relationship between density and strength of the latter.